As a professional photographer I use Phase One software to shoot studio shots and process them on a Macbook Pro. My cameras and software worked fine on my previous Macbook with Snow Leopard, and even my older cameras currently perform without fault on my G5 even with the pre-intel processor (a combination that isn't even recommended by the Phase One software).
I cannot, to my frustration, get any transfer of images from camera to computer using a doner Macbook Pro with Mountain Lion running the guts. My Phase One software freezes and the computer gets unpleasantly hot. Many of my fellow snappers have also expressed frustration with upgrade incompatibilities.
There are rather a lot of people like myself on the planet trying to make a living taking pictures and reliant on computers and software. Having to consider a downgrade to a previous operating system in order to keep things running does not make new upgrades seem to be in the consumers interest.